18 BULLETIN 1408, U. S. DEPARTMENT OF AGRICULTURE 
railway embankment near the outlet of a stream draining the back 
area. The gates are made of heavy timber, but the structure itself 
consists of two concrete sluiceways each 7.5 feet wide, 10 feet high, 
and 85 feet long, these large openings being necessary to permit pas- 
sage of logs. This sluiceway is supported with piling. 
OCEAN OUTLETS 
Some drainage districts are so situated that their outlets must 
discharge directly into the ocean (pi. 2, E). Such structures are 
continually subject to wave action intensified at times by storms. 
The profile in Figure 13 shows the conditions existing at the outlet 
of the Oxnard drainage district, Ventura County, Calif. 
Tide records and the tide tables of the United States Coast and 
Geodetic Survey should be studied in planning ocean outlets. The 
mean rise and fall of spring tide varies from 5.1 feet at San Diego, 
Calif., to 7.7 feet at Astoria, Oreg., with extreme variation several 
feet more than this. The elevation of mean lower low water is 2.9 
feet below mean sea level at San Diego and increases to 4.6 feet at 
Astoria. The elevation of the invert of outlet end of sluices will de- 
pend upon the elevation of land to be protected, tidal range, and 
difficulties of construction, but it should usually be below mean 
lower low water. The elevation of the Oxnard outlet is 4 feet be- 
low mean lower low water or 6.9 feet below mean sea level, United 
States Coast and Geodetic Survey datum. The extreme low tides of 
the year go below this, but usually the full ebb does not. 
All possible information should be obtained regarding the per- 
manence of the shore line and the soil through which the outlet pipe 
will pass. After the installation of the Oxnard outlet the loose for- 
mation at the outer end was shifted by wave action, with the result 
that the piling and outer end of the pipe were washed away. 
The end of the pipe should be carried out until the invert is about 
2 or 2.5 feet above the ground surface. If it is carried out too far 
the difficulty of holding the structure in place will be increased, 
whereas too short a distance will permit sand to be washed in. 
Because of ease of installation and flexibility, corrugated-iron cul- 
vert pipe, heavily galvanized, is well adapted to construction of the 
outer end; light cast iron has longer life. The sand-covered part 
of the conduit may be any strong, durable type. Besides the difficulty 
of holding the outer end in place some trouble may be experienced 
in making the pipe joints air tight, or as nearly so as is possible. 
Wave action may cause air and water to flow through weak joints, 
sucking sand into the pipe or completely uncovering it so that the 
entire structure may be endangered at high tide. Where corrugated 
pipe is used the joints at the outer end should be secured with a band 
not less than five corrugations wide equipped with pull lugs to 
insure adequate clamping. The safest method of protecting those 
joints entirely covered with sand is to surround them with concrete. 
Figure 13 shows a good design for the supporting structure. 
Square reinforced concrete piles are used. This form is easily made 
and offers greater clamping surface for the caps. The wooden caps 
should be creosoted and the pipe must be slung from the caps. Never 
less than two pipe lines should be installed, so that if one is dam- 
aged the other may operate while repairs are being made. Two 
